1. Field of the Invention
This invention relates to an assembly for protecting an X-ray head from the heat generated in a vacuum XRF film metrology system. In particular the invention relates to a containment vessel that protects the X-ray head while allowing X-rays to penetrate through a port in the vessel to reach the substrate, and enter back through the port to a detector in the head assembly.
2. The Prior Art
X-ray based metrology tools are required to manufacture the active layer in copper indium gallium diselenide photovoltaic cells (CIGS PV cells) that convert sunlight to electricity. Vacuum-based processes that deposit CIGS films must be controlled in a vacuum environment to ensure acceptable commercial output. Venting to air for process control would destroy the solar material. As a result, X-ray fluorescence (XRF) analysis of the integrity of the film stack (thickness and composition determination) must be performed in a vacuum. This allows adjustment of the deposition process tool to maintain engineered film tolerances. Vacuum-based CIGS PV manufacture requires real-time information that allows correction of process deviation immediately without losing vacuum to eliminate final PV film conversion efficiency output losses. XRF measurement data is required to manage yield and optimize conversion efficiencies for maximum resultant electric output of the PV material.
X-ray based metrology tools are expensive and delicate instruments that will not survive in a high temperature (500° C.) vacuum environment typical of internal CIGS and related film deposition chambers. The XRF tool must reside outside the deposition chamber to ensure tool survival and measurement capability. Typical stainless steel wall thicknesses for process deposition tools are about 0.5 to 1.0 inch, which will not allow X-ray frequency range radiation at the fluorescent power levels used in film metrology to penetrate these wall thicknesses.